SIGNIFICANCE OF PULSE RATE BUCHANAN. 497 



sources (3) and (16), weighs as much as 1.2 per cent of the body 

 weight, and is therefore rehitively larger than that of the mouse. A 

 very small dormouse on the other hand, in which the carbon-dioxide 

 output may be as much as 20.4 grams per kilo per hour when awake 

 (16), we should expect to have a pulse rate of over 1,000 per minute, 

 even if it has as large a heart (relatively) as the bat.^ It may have 

 a pulse rate as slow as 16 or 14 per minute when hibernating (16a). 

 Before going further a few words should be said about the method 

 of ascertaining the frequency of the beat in small warm-blooded 

 animals. It would be difficult to count a frequency of over 300 a 

 minute, or to record any mechanical movements of the lieart when 

 they are so rapid, in the living intact animal. We can, however, 

 make use of the fact, the meaning of which is not yet sufficiently un- 

 derstood (5) and (6), that the electrical changes accompanying all 

 muscular activity, and therefore that of the heart, produce in the case 

 of hearts of mammals, birds, and certain if not all reptiles, two electric 

 fields, the one of which pervades the anterior, the other the posterior 

 part of the body. In order to record the rate at which the fields 

 appear and disappear, we select some spot in each, e. g., the mouth 

 and one of the hind legs, and with some good conductor of elec- 

 tricity (such as wool or thread soaked in salt water) connect each 

 with a basin of salt water, these in their turn being connected with 

 the terminals of an instrument sensitive enough to record such 

 small differences of potential as come into existence between the two 

 fields. Such an instrument is the capillary electrometer represented 

 diagrammatically in figTire 2, which shows a bird ready to have its 

 pulse rate recorded. The instrument consists essentially of a fine 

 glass tube drawn out so as to be only a few thousandths of a milli- 

 meter in diameter near the tip, and filled with mercury. The open 

 end of the capillery tip dips into dilute sulphuric acid which enters 

 so far as the mercury permits, the tube being very slightly conical 

 so as just to prevent the mercury running out however near it be 

 to the tip. The properties of the instrument are such that if the 

 mercury becomes (galvanometrically) positive to the acid it moves 

 toward it, if negative it moves in the opposite direction. Since the 

 one field always comes into existence before the other, even though 

 it may be by no more than a thousandth of a second, there is always 

 a quick movement of the mercury in one direction while the single 

 field exists. There may be other movements, but these first quick 



^Note added in 1911. — Observations subsequently made by the author showed that an 

 ordinary sized dormouse has a pulse rate of 600 to 700 when awake and warm and a 

 heart weight which is about 1.2 per cent of the body weight ; also that the pulse rate of 

 a bat when awake is very variable, being in the very small form Natinugo pipistrellus now 

 about 200, now about 900 a minute, now something between, wliile in the larger form 

 Plecotus auritus, a specimen weighing 9.4 grams, had a pulse rate varying from 600 to 

 900 a minute (see 6a, and Troc. Physiol. Soc, Mar. 18, 1911). 



97578°— SM 1910 32 



